The present invention relates to a method for heat-treating compound semiconductor substrates, compound semiconductor epitaxial thin films and the like and, more particularly, to a method for heat-treating using an infrared lamp.
When an impurity which is introduced into a substrate of a III-V compound semiconductor such as gallium arsenide by ion implantation or the like is to be used as a carrier, a heat treatment called "activation" required for restoring crystal defects which arise on the ion implantation and causing impurity atoms thus introduced to move to desired lattice sites. In general, the temperature of this heat treatment is higher than the evaporating temperature of arsenic, an element constituting a gallium arsenide substrate. Accordingly, the heat treatment causes arsenic molecules to evaporate from the surface of the semiconductor substrate to form arsenic vacancy therein. This arsenic vacancy raises problems of a decreased activation rate and unevenness of the semiconductor substrate surface. Further, the gallium remaining on the substrate surface causes a problem of a variation in electronic device characteristics. There have been proposed approaches to prevent such evaporation of arsenic molecules, including cap anneal method, anneal method controlling ambient atmosphere, and a method in which the surface of a substrate is brought into contact with a susceptor.
Of these approaches the most convenient and effective one is the method comprising the steps of disposing a susceptor so as to be contacted with a surface of the compound semiconductor substrate in which an ion is implanted, and heat-treating the substrate. Particularly when a gallium arsenide substrate is heat-treated by the lamp anneal method employing an infrared lamp as a heating source, such a susceptor is indispensable that the gallium arsenide substrate scarcely absorbs infrared ray, and absorbs infrared ray instead to heat the substrate by heat conduction. It is highly advantageous to utilize this susceptor in preventing evaporation of arsenic.
The lamp anneal method is a method for rapidly heating and cooling an object in a short time and, hence, a susceptor for use in the lamp anneal method is desired to exhibit the following characteristics:
(1) being physically and chemically stable at elevated temperatures as high as 1000.degree. C. or above; PA1 (2) being processable for realizing a highly planar surface to be contacted by a heated material; PA1 (3) having a small heat capacity and a high thermal conductivity; PA1 (4) absorbing infrared ray from a heating source efficiently; and PA1 (5) not absorbing elements evaporated from the heated material. PA1 (a) disposing a susceptor in a manner as to be disposed on a surface of said compound semiconductor with opposing each other, and PA1 (b) heat-treating said compound semiconductor, wherein said susceptor comprises a compound of nitrogen and a group III element.
A single-crystalline silicon substrate or porous carbon substrate has been used for a susceptor used in the conventional lamp anneal method. The single-crystalline silicon substrate is advantageous in that it is chemically stable at elevated temperatures at most 1000.degree. C. or above and is processable for obtaining a high planarity. However, when subjected to rapid heating and cooling, the single-crystalline silicon substrate may suffer deformation due to heat such as warp or surface defects such as microslip. This causes the susceptor comprising the single-crystalline silicon substrate to contact gallium arsenide substrate differently at every heat treatment, thus leading to problems of insufficient evenness in the plane of a gallium arsenide substrate and an unsatisfactory reproducibility among test pieces of gallium arsenide substrates. Further, since silicon has an absorption band which scarcely overlaps the wavelength region of a light beam of an infrared lamp serving as the heat source, the single-crystalline silicon substrate is low in heating efficiency and hence is unsuitable for rapid heating and cooling. Furthermore, the solid solubility of arsenic with respect to silicon is as high as 1.times.10.sup.21 /cm.sup.3, and when under a condition higher than that value, an arsenic compound is produced. Accordingly, there arises a problem that the single-crystalline silicon susceptor itself absorbs the arsenic evaporated from the surface of the gallium arsenide substrate.
On the other hand, the carbon substrate is excellent in that it absorbs infrared ray in the wavelength region of a light beam of the infrared lamp, but involves a drawback of unsuitableness for rapid heating and cooling in a short time due to a large heat capacity thereof. Further, the carbon substrate is porous and hence serves to absorb the arsenic evaporated from the gallium arsenide substrate.
As described above, the single-crystalline silicon substrate or porous carbon substrate conventionally used for the susceptor for use in the lamp anneal method cannot be considered an optimum material for the susceptor. In addition, there has not been provided any other optimum materials for the susceptor.
It is, therefore, an object of the present invention to overcome the problem that the conventional heat treating method for a compound semiconductor such as gallium arsenide has not been provided with an optimum material for a susceptor, and to provide an excellent heat treating method using an optimum material for the susceptor.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description.